The lone remaining holdout is Sen. Blanche Lincoln (D-Ark.), who faces a difficult reelection race in her conservative home state next year. Sen. Ben Nelson (D-Neb.) withheld his support until Friday when, like Landrieu, he announced he would vote with his party Saturday while emphasizing he reserved the right to oppose the bill itself when the debate concluded or even participate in a Republican filibuster to prevent the measure from coming to a final vote.

At around 8 p.m. Saturday night, the Senate will vote on a procedural motion to proceed that will enable the upper chamber to kick of its long-awaited debate on legislation that would spend $848 billion over 10 years to extend health insurance coverage to 31 million more people while reducing the federal budget deficit by $127 billion, according to a Congressional Budget Office analysis.

Landrieu offered high praise for Reid's bill -- but emphasized she is not satisfied. The legislation, she said, "is the best work of the Senate to date on a issue of significant importance to the people of my state and the country."

But Democratic leaders still have to win her over fully if they expect her to support the underlying bill when the debate concludes. "My vote today to move forward on this important debate should in no way be construed by the supporters of this current framework as an indication of how I might vote as this debate comes to an end," Landrieu said.

In particular, Landrieu said she wants to see more assistance for small-business employees and the self-employed and that the health insurance benefits in the legislation begin sooner than the 2014 effective date set by the legislation.

Landrieu also said she continues to oppose the form of government-run public option contained in the bill. Reid proposed setting up a public option from which states could opt out. Landrieu, however, expressed interest in the so-called public option trigger, espoused by Republican Sen. Olympia Snowe (Maine), that would implement a public option only in states where private insurance fails to meet benchmarks for affordability and availability.

Although these centrists Democrats have insisted their votes to advance the bill on Saturday are not the same as full endorsements of the legislation itself, Republicans have consistently argued that the distinction is meaningless and that they will portray affirmative votes on procedural matters as equivalent to votes in favor of the bill.

Landrieu also pushed back on media reports that her support for the procedural motion is directly related to the fact that Reid added roughly $100 million for Louisiana's Medicaid program in the bill. Those provisions, Landrieu maintained, are designed to address lingering problems in her home state related to its recovery from the devastation of Hurricane Katrina.